Histamine is an autacoid, which means it acts similarly to a local hormone, near its site of synthesis. It is produced as part of the local immune response to invading bodies and triggers inflammation as well as performing several important functions in the bowel.

Sites of histamine release

• Much of the histamine in the body is produced by the granules in mast cells and basophils as part of a local immune response to the presence of invading bodies. The basophils and mast cells are found in nearby connective tissue and are particularly abundant at sites where injury may occur such as the nose, mouth, feet, surfaces of the internal organs and blood vessels. Histamine is also made in the brain where it acts as a neurotransmitter or chemical messenger that carries signals between nerves.
• Another important site where histamine is stored and released is in the enterochromaffin-like (ECL) cells of the stomach.

Effects of histamine

• Histamine is released at the mucosal surfaces as a result of exposure to foreign particles. This histamine release causes the capillaries to become more permeable to white blood cells, which move into the capillaries and proceed to target and attack foreign bodies in the affected tissue. The increased permeability of the capillaries causes fluid to move out of capillaries, which gives rise to the classic symptoms of allergy such as a runny nose and watery eyes. As the foreign antigens bind to IgE-sensitised mast cells in the mucous membranes, several responses occur. Sensory neural stimulation associated with the histamine release leads to sneezing; the glandular tissue secretes fluids and nasal congestion occurs due to the vascular engorgement caused by increased vasodilation and capillary permeability.
• Non-mast cell histamine is released in the brain where it acts as a neurotransmitter. The histamine neurons are found in the tuberomammillary nuclei of the posterior hypothalamus. From there, they extend throughout the brain into the cortex and medial forebrain bundle. These neurons increase wakefulness and also prevent sleep.
• In the stomach, histamine stimulates the parietal cells to produce the gastric acids required for digestion.